This invention relates to a control circuit with an auxiliary process variable.
Heretofore, various types of control circuits have been known for controlling the output of a plant. For example, Swiss Pat. No. 369,780 describes a circuit with an auxiliary process variable for controlling the output temperature of a working medium flowing through a heat exchanger. As described, the heat exchanger has an input for receiving the working medium, a meter for emitting a signal corresponding to the output temperature as a process variable of the working medium, a controller for receiving the signal, a final control element which is connected to the controller in order to control the medium delivered to the heat exchanger and a signal transmitter which is connected to the heat exchanger near the input for emitting a second signal in response to an auxiliary process variable of the working medium. The use of the auxiliary process variable greatly improves the quality of the control. However, it has been found that when the controller is set to the optimum value for this circuit, dangerously high deviations may occur in the event of an absence of the auxiliary process variable signal.
While process variables can, in most cases, be measured without difficulty and with little likelihood of failure, it is often difficult to fit a signal transmitter for an auxiliary process variable. In the case of the control circuit described in the above-noted Swiss Patent, the measuring point for the auxiliary process variable must be disposed inside a housing of the heat exchanger. As a result, the likelihood of failure is relatively high because of the unfavorable situation of the measuring point.
It is obvious that the increased liability of the signal transmitter to malfunction can be counteracted by providing a second, and possibly a third signal transmitter for the auxiliary process variable and to feed these signals from these transmitters to the controller by way of one of the known redundancy circuits. However, a solution of this kind is not only expensive but also increases the probability of a disturbance occurring which would result in the need for repairs and, possibly, shutdown ofthe plant housing the heat exchanger.
Accordingly, it is an object of the invention to improve a control circuit having an auxiliary process variable at moderate expense and with a minimum of failure probability.
It is another object of the invention to provide a control circuit for a plant in which an auxiliary process variable is monitored so that dangerous deviations are minimized in the event of failure of the auxiliary process variable signal.
It is another object of the invention to be able to modify existing control circuits which monitor auxiliary process variables in a relatively economic manner to avoid dangerous deviations which may occur in the event of failure of an auxiliary process variable signal.
Briefly, the invention is directed to a control circuit for controlling a plant having an input for receiving a medium to be processed therein. The control circuit includes a meter for emitting a signal corresponding to a process variable of the medium processed in the plant, a controller having an input connected to the meter for receiving the signal and an output for emitting a control signal, a final control element connected to the controller output to receive the control signal for controlling the medium delivered to the plant and a signal transmitter connected to the plant near the plant input for emitting a second signal in response to an auxiliary process variable of the medium. In accordance with the invention, the control circuit is provided with a selector, a function transmitter and a disturbance indicator.
The selector has a pair of inputs and an output. One of the selector inputs is connected to the signal transmitter in order to receive the auxiliary process variable signal while the output is connected between this selector input and the second controller input in order to deliver the signal thereto.
The function transmitter is connected to and between the controller output and a second selector input in order to deliver the control signal from the controller output to the selector.
The disturbance indicator is connected to and between the signal transmitter and the selector in order to switch the selector output to the selector input connected to the function transmitter in response to an output signal from the signal transmitter falling below a bottom threshold value or above a top threshold value. Upon actuation of the disturbance indicator, the controller is then influenced by the function transmitter instead of by the signal transmitter.
One particular advantage of the control circuit is that the selector, function transmitter and disturbance indicator can be incorporated in the controller itself. In this case, no additional connecting lines are required.
The control circuit can be constructed with different controllers. For example, in one embodiment, the controller includes a first network of at least I-type which is connected to the controller input receiving the signal from the meter, a second network of at least P-type connected to the controller input receiving a signal from the selector and a subtraction point connected to the networks to form a final control signal of the received signals in the networks. In this embodiment, the control circuit is particularly simple.
In another embodiment, the controller may have a network of at least I-type connected to the controller input which receives the signal from the meter, a subtraction point connected to the network to receive an integral action signal therefrom and to the selector output to receive a signal and a second network of at least I-type connected between the subtraction point and the controller output. A control circuit with a controller of this type offers additional adjustment facilities so that better optimization is possible.
In still another embodiment, the controller may include a P-network connected to and between the controller input receiving a signal from the meter and the controller output as well as a D-network connected to the selector output and to the P-network for feeding a derivative action signal negatively to the P-network corresponding to the signal from the selector. The advantage of this control circuit is that no provision has to be made to prevent a runaway of the controller output signal. However, one disadvantage of this control circuit is that it may lead to a permanent small deviation. In order to obviate this disadvantage, a PI-network can be connected to and between the plant output and the controller input receiving the process variable signal.